High-temperature oxidation-resistant cobalt base alloys

ABSTRACT

Cobalt base alloys having improved high-temperature strength, ductility, and oxidation resistance consist essentially of, in percent by weight, chromium 20-35, carbon 0.05-1.5, tungsten 212, tantalum an effective amount of about 1 up to 7, iron 3-17, boron an effective amount of about 0.005 up to 0.1, yttrium 0.050.4, titanium an effective amount of about 0.1 up to 3, zirconium an effective amount of about 0.1 up to 3, with the remainder essentially cobalt except for impurities.

United States Patent Adrian M. Beltran;

Chester T. Sims, both of Ballston Lake, N.Y.

Dec. 30, 1969 Nov. 2, 197 1 General Electric Company Inventors Appl. No.Filed Patented Assignee HIGH-TEMPERATURE OXIDATION-RESISTANT COBALT BASEALLOYS [56] References Cited UNITED STATES PATENTS 3,202,506 8/1965Deutsch 75/171 Primary Examiner-Richard 0. Dean Attorneysl-larold J.Holt, William C. Crutcher, Frank L. Neuhauser, Oscar B. Waddell andJoseph B. Forman ABSTRACT: Cobalt base alloys having improvedhigh-temperature strength, ductility, and oxidation resistance consistessentially of, in percent by weight, chromium 20-35, carbon 0.05-L5,tungsten 2-12, tantalum an effective amount of about 1 up to 7, iron3-17, boron an effective amount of about 0.005 up to 0.1, yttrium0.05-0.4, titanium an effective amount of about 0.1 up to 3, zirconiuman effective amount of about 0.1 up to 3, with the remainder essentiallycobalt except for impurities.

HIGH-TEMPERATURE OXIDATION-RESISTANT COBALT BASE ALLOYS This inventionrelates to new and useful cobalt base alloys which are particularlycharacterized by improved high-temperature strength and ductility andhave increased resistance to oxidation and hot corrosion at elevatedtemperatures.

Gas turbines and other equipment which depend upon the driving force ofcombustion gases operated more efficiently as the operating temperaturerises. However, at such higher temperatures the strength of many alloysneeded for both rotating andstationary parts often decreases rapidly,and the alloys become susceptible to oxidation caused by contact withthe hot combustion gas stream. These conditions have precipitated aconstant search for new and improved alloys and even relatively smallimprovements in high-temperature strength and oxidation resistancebecome very important. In gas turbines operating at temperatures of theorder of about l,600 F. with a peak of about 2,000 F., an improvement ofonly about 100 F. in the oxidation or corrosion resistance of thestructural materials such as buckets, partitions, and other componentsrepresents a notable advance. For example, an increase in operatingtemperature of a typical gas turbine from about 1,500 to l,600 F.produces an increase in power output of about 14 percent and an increasein efficiency of up to 1 about percent. The constant search for suchhigh-temperature alloys will thus be appreciated, and it is a principalobject of this invention to provide new and useful alloys which willpermit the operation of equipment such as gas turbines at temperaturesof up to about 1,900" to 2,000 F. or even higher. Another object of theinvention is to provide improved materials of construction forhigh-temperature equipment in general which are subjected to oxidativeatmospheres such as furnaces and the like.

Those features of the invention which are believed to be novel are setforth with particularity in the claims appended hereto. The inventionwill, however, be better understood and further objects thereofappreciated from a consideration of the following description.

Briefly, there are provided by the present invention economical,high-temperature, oxidation-resistant, cobalt base alloys which are alsocharacterized by good room temperature and elevated temperature strengthcharacteristics and goodhot corrosion resistance which have a percent byweight composition of chromium -35, carbon 0.05-l.5, tungsten 2-12,tantalum an effective amount of about 1 upto 7, iron 3-17, boron aneffective amount of about 0.005 up to 0.1, yttrium ODS-0.4, titanium aneffective amount of about 0.1 up to 3, zirconium an effective amount ofabout 0.1 up to 3, with the remainder essentially cobalt except forimpurities such as manganese, silicon, sulfur, and phosphorus.Preferably, the manganese is kept below a maximum of about 1.2 percent,the silicon below about 1 percent, and the sulfur and phosphorus eachbelow about 0.04 percent.

It has been found that alloys of the above precisely balancedcomposition are characterized by substantial improvements in oxidationresistance at elevated temperatures, at the same time retaining suitablestrength, ductility, and other physical characteristics for operation atsuch temperatures. The alloys are also particularly useful in that theyare adapted to precision investment casting techniques and other moldingtechniques which permit the precision formation of various shapedstructures suitable for high-temperature apparatus such as buckets andsuch of the hot stages of gas turbines.

Those features of the invention which are believed to be patentable areset forth with particularity in the claims appended thereto. Theinvcntion will, however, be better understood and further advantages andobjects thereof appreciated from a consideration of the followingdescription.

It is to be particularly noted that the relatively high range ofchromium of from about 20 to weight percent is contrary to general priorart teaching that cobalt base alloys having a chromium content of overabout 25 percent by weight show increased scaling or deterioration dueto oxidative or corrosive influence at elevated temperatures. Such priorart teaching is set forth, for example, inJournal of the ElectrochemicalSociety, Vol. 103, No. 8, by Pfalnikar et al. entitled High TemperatureScaling of Cobalt-Chromium Alloys."

The present compositions represent a carefully balanced formulation ofconstituents, each of which contributes in the amounts stated to thedesirable end results obtained. Deviations in the amounts of materialsdestroy this critical balance resulting in materials which have beenfound to be lacking in one or more desired characteristics. For examplereduction of the chromium content below that prescribed results in adetrimental loss of oxidation resistance while excessive amounts ofchromium produce precipitation of a cobalt plus chromiumrich sigma phaseintermetallic compound, which precipitation embrittles the alloy duringservice and further renders it brittle at room temperature. When thecarbon is lowered beyond that indicated, undesirable weakening occurs,whereas increasing the carbon content above that set forth results in anembrittling tendency due to excessive precipitation of metal carbides atthealloy grain boundaries. Lesseramounts of tungsten than those statedresult in weakening, as tungsten substitutionally solid-solutionstrengthens the alloy matrix lattice. Amounts greater than those setforth again result in embrittlement, as tungsten' enhances precipitationof sigma phase. Tantalum, titanium and zirconium are necessary asstrengthening constituents through formation of the cubic carbidestructure, previously enriched in tantalum but also containing titaniumand zirconium. In excess of the prescribed ranges of these elements, anundesirable amount of MC carbide precipitator creates an imbalance withthe second major strengthening carbide, chromium-rich Cl'gaCq-Furthermore, an undesirable reaction occurs between the MC carbide andthe ceramic mold when the molten alloy is poured during the castingoperation.

It is to be noted here that it has been the practice in the prior art touse nickel as a matrix stabilizer. However, it is a critical finding ofthis invention that iron, which is much less. expensivethan nickel,effectively stabilizes the alloy, matrix. That is, an equal percentagesubstitution of iron for nickel more effectively inhibits transformationof the matrix crystallographic structure from the high-temperature facecentered cubic polymorph to the low-temperature, less ductile hexagonalclosely packed polymorph. Further, nickel is in world. wide short supplywhile iron is widely available, which increases the practicality of thisalloy immensely while at the same time sharply decreasing the price.Greater amounts of iron than those set forth, however, unduly weaken thealloy.

in the prescribed range, boron strengthens the alloy throughprecipitationof metal borides and creation of thermodynamic grainboundary perfection. in excess amounts, howcvenmetal borideprecipitation at the alloy grain boundaries severely embrittles thealloy. Yttrium is particularly critical to the oxidation and hotcorrosion resistance of these alloys, by the manner in which propertiesof the predominant oxide, C50 are improved. Adherence of this scale,particularly under thermal cycling conditions, is markedly improved dueto the mechanical keying of scale to alloy substrate afforded by thepresence of yttrium-rich oxide particles formed near the oxidizingsurface. These same particles inhibit the free flow of chromium atoms tothe surface, thereby reducing the rate at which the alloy oxidizes.Greater amounts of yttrium, however, lead to formation of an yttriumand,cobalt-rich intermetallic compound, which embrittlesthe alloy. As apractical matter, furthermore, it is quite difficult to retain greateramounts of yttrium during the casting operation because of its extremereactivity and subsequent loss to the slag, Amounts of manganese andsilicon over those prescribed result in unwanted embrittlement andweakening as a result of sigma phase formation, or of otherintermetallic compounds whose formation is enhanced in particular bysilicon.

The following examples will illustrate the practice of the invention, itbeing realized that they are exemplary only and not to be taken aslimiting in any way.

EXAMPLE 1 There was prepared by vacuum induction melting techniques analloy consisting of by weight percent: chromium 24, carbon 0.65,tungsten 7, tantalum 3.5, iron 10, boron 0.015, yttrium 0.15, titanium0.2, zirconium 0.5, manganese 0.3, silicon 0.1, sulfur 0.015, andphosphorus 0.015, with the remainder essentially cobalt except for otherincidental impurities. This alloy was poured into ceramic molds toprepare test bars 3 inches long by 0.252 inch diameter. A first heat,heat No. I, had a casting temperature of 2,850 F., a mold temperature of1,500 F., and was cooled in the enclosed mold. Heat No. 2 had a castingtemperature of 2,680 F., a

Shown in table ll is the hot corrosion resistance of the presentexemplary alloy as compared to the above prior art alloy. In this test,disc-shaped test pieces of the, above example and the prior art materialwere placed in the combustion gas stream flow in a simulated gasturbineburner apparatus at the temperatures indicated using natural gasas a fuel at an air-tofuel weight ratio of 50 to l. TI-Ie specimens werethermal cycled" every 50 hours to simulate gas turbine shutdown, thisprocedure being particularly rigorous-as it evaluates the adherenceproperties of the protective scale. After the times indicated, thesurface loss and maximum-penetration were measured metallographicallyfor each sample in terms of mils per side.

TABLE 11 Maximum Surface Temp, Time, penetration, loss, Ex. Heat Fuel F.hrs. mils mils 1 1 Natural gas 1, 800 630 3. 1. 2 do........ 2,000 6209.9 4.7 Natural gas. 1, 800 606 4. 0. 0 .do... 1 1,900 606 5. 3 2.0 2 1do. 1 2, 000 619 10. 5 7. 1 Diesel 011........... 1,600 028 2.1 0.5

and Sea salt 1, 800 017 5. 5 0. 6 Natural gas. 1, 800 600 4. 1. ....(11,900 600 1g. 0 2,000 600 (2) Diesel 011 1, 600 600 2. 2 0. 5

and Sea salt 1,800 600 4 8 1.4

1 Average of 2 tests. 1 Prior an alloy. average results.

mold temperature of 1,800 F., and again was cooled in the enclosed mold.Heat No. 3 had a casting temperature of 2,680 F., a mold temperature ofl,800 F., and was cooled in air, the mold being broken open aftersolidification of the The oxidation resistance of the prior art alloy,while acceptable at l,800 F. in natural gas, rapidly worsens withincreasing temperature. The present exemplary alloy, therefore, isdistinctly more resistant at the higher temperatures. ln diesel mclloilplus sea salt combustion products, the two are virtually EXAMPLE 2equivalent at both 1,600 and l,800 F., this being superior to There wasprepared by vacuum induction melting most contemporary bflsc techniquesan alloy consisting of, by weight percent, chr From the above table itWlll be quite evident that the present um 24.8, carbon 0.68, tungsten6.6, tantalum 3.64, iron 9.0, alloys which are Characterized by muchImproved rupture boron 0.015, yttrium 0.22, titanium 0.2, zirconium 0.5,manductility at elevated temperatures far and y Superlol' ganese 0.1,silicon 0.1, sulfur 0.015, and phosphorus 0.015, oxidation resistance atelevated temperatures to the prior art with the remainder essentiallycobalt except for other iny; such Oxidation resistance combined wlih thenaturally cidema] impurities Test bars 3 inches long b 25 i h iexcellent hot corrosion resistance of cobalt alloys make these diameterwere prepared by vacuum induction melting and inalloys y useful forOperation under high-temperature oxlda' vestment casting techniques.tion conditions which are experienced in gas turbine and Shown in tableI are the high-temperature stress-rupture similar apparatus. The alloysare considered as particularly atproperties of the examples of the alloyclaimed as compared tractive for gas turbine bucketing where such highstrength is with a typical prior art alloy, specifically Mar M-509.mandatory, usually preserved for nickel-base alloys.

The Larsen-Miller parameter (constant =20) is a well- What we claim asnew and desire to secure by Letters known numerical value which combinestime and temperature Patent ofthe United States is: to allow comparisonof the capabilities of alloys on a normal- I. A cobalt base alloycharacterized by good high-temperaiZed basis, Such that at y giventemperature e parameter ture strength and ductility and corrosionresistance consisting gives a direct comparison of ruptur tr ngt aluessentially of about, by weight, chromium 20-35 percent, car- TABLEI.STRESS-RUPTURE TESTS Larsen- Percent Percent Miller Temp., Stress,Life, elongareduction parameter Alloy F. K s.i. hours tion in area (020) 1,600 25 101.6 19.4 49 45.3 Heat 1 1,800 15 40.8 18.4 34 48.8 2,0009 11.4 23.9 43 51. 8 1, 600 25 59.5 27.4 52 44. 8 Heat 2 1,800 15 27.241.3 72 48.5 2, 000 0 4. 8 39. 9 50. 9 1,600 25 249.5 28.7 09 46.2 Heat3 1,800 15 79.3 35.3 70 49.5 2,000 9 19.7 56. 2 s4 52. 4 l,600 25 205.015.0 25 46.0 Prior art (typical) 1, 800 15 80. 0 15. 0 25 49.5 000 9 35.0 8.0 15 53. 0

From the above table it will be seen that the materials of the presentinvention have about two times the ductility of the prior art materialas measured by reduction in area. The stress-rupture test results intable I indicated that the stressrupture strength of present alloysusing the Larsen-Miller parameter indicated has a strength comparable tothe prior art alloys at high stresses.

bon 0.05-l.5 percent tungsten 2-12 percent, tantalum an effective amountof about 1 percent up to 7 percent, iron 3-17 percent boron an effectiveamount of about 0.005 percent up to 0.1 percent yttrium 0.05-0.4 percenttitanium an effective amount of about 0.1 percent up to 3 percent,zirconium an effective amount of about 0.1 percent up to 3 percent, withthe remainder essentially cobalt except for impurities.

consisting essentially of, by weight, chromium 24.8 percent. carbon 0.65percent, tungsten 6.6 percent, tantalum 3.64 percent, iron 9.0 percent,boron 0.0l5 percent. yttrium 0.22 percent, titanium 0.2 percent,zirconium 0.5 percent, manganese 0.1 percent, silicon 0.1 percent,sulfur 0.015 percent. and phosphorus 0.015 percent, with the remainderessentially cobalt'except for incidental impurities.

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2. A cobalt base alloy as in claim 1 characterized by goodhigh-temperature strength and ductility and corrosion resistanceconsisting essentially of about, by weight, chromium 24 percent, carbon0.65 percent, tungsten 7 percent, tantalum 3.5 percent, iron 10 percent,boron 0.015 percent, yttrium 0.15 percent, titanium 0.2 percent,zirconium 0.5 percent, with the remainder essentially cobalt except forimpurities.
 3. A cobalt base alloy as in claim 1 characterized by goodhigh-temperature strength, ductility and corrosion resistance consistingessentially of, by weight, chromium 24.8 percent, carbon 0.65 percent,tungsten 6.6 percent, tantalum 3.64 percent, iron 9.0 percent, boron0.015 percent, yttrium 0.22 percent, titanium 0.2 percent, zirconium 0.5percent, manganese 0.1 percent, silicon 0.1 percent, sulfur 0.015percent, and phosphorus 0.015 percent, with the remainder essentiallycobalt except for incidental impurities.